Boris Johnson questioned whether the logic for Brexit was "wholly correct" in a never before published article just days before he came out supporting Britain leaving the EU.
The secret column was drafted by the now foreign secretary in February and argued that the implications of Brexit on relationships with Scotland and Russia needed to be considered.
He also mulled over the "economic shock" expected by "everyone", asking: "How big would it be? I am sure that the doomsters are exaggerating the fallout — but are they completely wrong? And how can we know?"
The then Mayor of London wrote that the cost of being a member of the union was small for what the country gets in return in the column, revealed by the Sunday Times.
"Britain is a great nation, a global force for good. It is surely a boon for the world and for Europe that she should be intimately engaged in the EU. This is a market on our doorstep, ready for further exploitation by British firms: the membership fee seems rather small for all that access," he wrote.
He also said: "Shut your eyes. Hold your breath. Think of Britain. Think of the rest of the EU. Think of the future. Think of the desire of your children and your grandchildren to live and work in other European countries; to sell things there, to make friends and perhaps to find partners there."
Johnson wrote the column after he had already penned an article backing Brexit and did so "as a way of clarifying his thoughts", according to a new book by Sunday Times political editor Tim Shipman, All Out War.